The simplest way to get a ticket is to go straight to RHI Reception (IN009, International Bldg.) or the Music Department Office in Wettons Terrace. You will need to let the office staff know what dish you are bringing to the feast.
If you are unable to go in person to these departmental offices, you can apply for a ticket online here. Fill out the fields on the Googledocs form and submit. Wait for email confirmation that you have secured a ticket, and then pick up your ticket from RHI Reception (IN009) - or at the International Music Feast itself.
Tickets are allocated on a strictly first come, first served basis.